In a gasoline engine, over 99% of the emissions that come out of the engine are converted to clean output (H2O and CO2) by the catalyst in the catalytic converter. However, to operate properly, the catalyst must be heated to a temperature equal to or above a light-off temperature. The light-off temperature is approximately equal to 400° C. The majority of the exhaust gas emissions that are captured exiting the exhaust gas treatment system during an emissions test occur during the first 60 seconds of engine operation while the catalyst temperature is below the light-off temperature, and can not efficiently convert the engine out emissions to H2O and CO2.
Conventional vehicle powertrains typically will execute a Catalyst Light Off (CLO) strategy immediately upon starting the engine when the drivetrain is in park or neutral and hope to finish before the driver tips into the accelerator pedal and drives away. This way the focus of CLO strategy can be on the emissions performance of the engine and not the drivability of the vehicle. Some hybrid powertrain combinations can achieve the maximum acceleration that the driver requests with the electric motors by themselves. These combinations can often start the engine, with the engine decoupled from the drivetrain, and perform the CLO strategy in a way that is much like the conventional powertrain in terms of focus on emissions. However, many hybrid powertrain combinations have an engine that provides a major portion of the vehicles propulsion. These vehicles will often use a feature called “Silent Start” where the engine is not started as soon as the driver enables the propulsion system. Instead the engine will be started some time later when the driver is in drive (drivetrain engaged) and the vehicle is moving down the road.